Power supply system for electrically powered vehicle and method for controlling the same

ABSTRACT

A power supply system includes a main power storage device and a plurality of sub power storage devices. A converter is connected to selected one of the sub power storage devices to convert voltage between the selected sub power storage device and an electric power feeding line bidirectionally. When a request for switching the selected sub power storage device in use is generated, a converter steps up voltage on the electric power feeding line to a predetermined voltage, and thereafter a process for switching relays is performed. The predetermined voltage is higher than any of a voltage output from the main power storage device and a voltage output from a sub power storage device to be newly connected to the converter. Further, during the process for switching connection of the relays, upper limits on electric power input/output to/from the selected sub power storage device are set to 0.

TECHNICAL FIELD

The present invention relates to a power supply system for anelectrically powered vehicle and a method for controlling the same, andmore particularly to control of a power supply system for anelectrically powered vehicle equipped with a main power storage deviceand a plurality of sub power storage devices.

BACKGROUND ART

In recent years, electrically powered vehicles such as electric cars,hybrid cars, fuel cell cars, and the like have been developed intopractical use as environmentally friendly vehicles. These electricallypowered vehicles are each equipped with an electric motor generatingforce to drive the vehicle, and a power supply system configured toinclude a power storage device for supplying electric power to drive theelectric motor.

In particular for hybrid cars, there has been proposed a configurationcharging a vehicle-mounted power storage device by a power supplyexternal to the vehicle (hereinafter also referred to as an “externalpower supply”). Accordingly, these electrically powered vehicles havebeen required to have an increased distance travelable using electricpower stored in the vehicle-mounted power storage device. Hereinafter,charging of a vehicle-mounted power storage device by an external powersupply will also be referred to simply as “external charging”.

For example, Japanese Patent Laying-Open No. 2008-109840 (PatentDocument 1) and Japanese Patent Laying-Open No. 2003-209969 (PatentDocument 2) describe a power supply system having a plurality of powerstorage devices (batteries) connected in parallel. The power supplysystem described in Patent Documents 1 and 2 includes a voltageconverter (a converter) provided for each power storage device (battery)for serving as a charging/discharging adjustment mechanism. In contrast,Japanese Patent Laying-Open No. 2008-167620 (Patent Document 3)describes a configuration of a power supply device in a vehicle equippedwith a main power storage device and a plurality of sub power storagedevices, the power supply device including a converter associated withthe main power storage device and a converter shared by the plurality ofsub power storage devices. This configuration can achieve a reducednumber of elements of the device and also an increased amount of energythat can be stored.

-   Patent Document 1: Japanese Patent Laying-Open No. 2008-109840-   Patent Document 2: Japanese Patent Laying-Open No. 2003-209969-   Patent Document 3: Japanese Patent Laying-Open No. 2008-167620

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the power supply device described in Patent Document 3, one of theplurality of sub power storage devices is selectively connected to theconverter to allow the main power storage device and the selected subpower storage device to supply electric power to drive an electric motorfor driving a vehicle. In such a power supply device, when the SOC(State of Charge) of the sub power storage device in use decreases,another sub power storage device is newly connected to the converter. Insuch a manner, the plurality of sub power storage devices are usedsequentially, thereby increasing a travelable distance (EV (ElectricVehicle) travelable distance) achieved by stored electric energy.However, at the time of switching connection of the sub power storagedevice, it is necessary to perform a switching process through anappropriate process procedure to avoid failure in equipment due tooccurrence of a short-circuit path, and the like.

The present invention has been made to solve such problems, and oneobject of the present invention is to appropriately perform, in a powersupply system configured to include a main power storage device and aplurality of sub power storage devices, and a voltage converter (aconverter) shared by the plurality of sub power storage devices, aconnection switching process for changing a sub power storage device tobe used.

Means for Solving the Problems

According to the present invention, a power supply system for anelectrically powered vehicle equipped with a motor generating power todrive the vehicle includes a main power storage device, an electricpower feeding line, a first voltage converter, a plurality of sub powerstorage devices provided in parallel to each other, a second voltageconverter, a connection unit, and a switching control device. Theelectric power feeding line is configured to supply electric power to aninverter that controls and drives the motor. The first voltage converteris provided between the electric power feeding line and the main powerstorage device, and configured to convert voltage therebetweenbidirectionally. The second voltage converter is provided between theplurality of sub power storage devices and the electric power feedingline, and configured to convert voltage between one of the plurality ofsub power storage devices and the electric power feeding linebidirectionally. The connection unit is provided between the pluralityof sub power storage devices and the second voltage converter, andconfigured to selectively connect a sub power storage device selectedfrom the plurality of sub power storage devices to the second voltageconverter. The switching control device is configured to controlselective connection between the plurality of sub power storage devicesand the second voltage converter. The switching control device includesa switching determination unit, a first electric power limiter unit, aconnection switching control unit, and a second electric power limiterunit. The switching determination unit is configured to determinewhether or not the selected sub power storage device should be switchedbased on states of charge of the plurality of sub power storage devices.A step-up-voltage instruction unit is configured to instruct the firstvoltage converter to provide a voltage on the electric power feedingline to be a first voltage higher than a voltage output from the mainpower storage device and a voltage output from a sub power storagedevice to be connected to the second voltage converter after switching,when the switching determination unit determines that it is necessary toswitch the selected sub power storage device. The first electric powerlimiter unit decreases values of upper limits on electric powerinput/output to/from the selected sub power storage device gradually tozero after the voltage on the electric power feeding line has reachedthe first voltage. The connection switching control unit is configuredto switch connection between the plurality of sub power storage devicesand the second voltage converter, when the first electric power limiterunit sets the values of the upper limits on electric power input/outputto zero. The second electric power limiter unit is configured toincrease the values of the upper limits on electric power input/outputgradually to values corresponding to a state of charge of the sub powerstorage device newly connected to the second voltage converter after theconnection switching control unit switches connection between theplurality of sub power storage devices and the second voltage converter.

Alternatively, in a method for controlling a power supply system for anelectrically powered vehicle according to the present invention, thepower supply system includes the main power storage device, the electricpower feeding line, the first voltage converter, the plurality of subpower storage devices, the second voltage converter, and the connectionunit described above. The method for controlling includes the steps ofdetermining whether or not the selected sub power storage device shouldbe switched based on states of charge of the plurality of sub powerstorage devices, instructing the first voltage converter to provide avoltage on the electric power feeding line to be a first voltage higherthan a voltage output from the main power storage device and a voltageoutput from a sub power storage device to be connected to the secondvoltage converter after switching, when the step of determiningdetermines that it is necessary to switch the selected sub power storagedevice, decreasing values of upper limits on electric power input/outputto/from the selected sub power storage device gradually to zero afterthe voltage on the electric power feeding line has reached the firstvoltage, switching connection between the plurality of sub power storagedevices and the second voltage converter when the step of decreasingsets the values of the upper limits on electric power input/output tozero, and increasing the values of the upper limits on electric powerinput/output gradually to values corresponding to a state of charge ofthe sub power storage device newly connected to the second voltageconverter after the step of switching switches connection between theplurality of sub power storage devices and the second voltage converter.

Preferably, the step-up-voltage instruction unit continues to instructthe first voltage converter to provide the voltage on the electric powerfeeding line to be the first voltage until a process for increasing thevalues of the upper limits on electric power input/output by the secondelectric power limiter unit ends. Alternatively, the method forcontrolling further includes the step of continuing to instruct thefirst voltage converter to provide the voltage on the electric powerfeeding line to be the first voltage until a process for increasing thevalues of the upper limits on electric power input/output in the step ofincreasing ends.

Preferably, the first voltage corresponds to a value of an upper limiton the voltage on the electric power feeding line controlled by thefirst voltage converter.

According to the power supply system for an electrically powered vehicleand the method for controlling the same described above, at the time ofswitching connection between the second voltage converter and a subpower storage device, voltage on the electric power feeding line isstepped up to the first voltage higher than any of the voltage outputfrom the main power storage device and the voltage output from a subpower storage device to be newly used, and thereafter the sub powerstorage device to be newly used can be connected to the second voltageconverter. This can prevent formation of a short-circuit path from thesub power storage device to be newly used via the electric power feedingline. Further, the values of the upper limits on electric powerinput/output to/from the sub power storage device are decreased beforeswitching connection of the sub power storage device, and the values ofthe upper limits on electric power input/output are caused to returngradually after completion of switching of connection. This can preventthe power supply system from being requested to excessivelycharge/discharge electric power in a period in which electric powercannot be input/output to/from the sub power storage device due toswitching of connection.

Preferably, the switching control device further includes a thirdelectric power limiter unit, and the third electric power limiter unitis configured to temporarily relax charging and discharging limits forthe main power storage device in a period from when the first electricpower limiter unit starts decreasing the values of the upper limits onelectric power input/output to when the connection unit completesswitching of connection between the plurality of sub power storagedevices and the second voltage converter. Alternatively, the method forcontrolling further includes the step of temporarily relaxing chargingand discharging limits for the main power storage device in a periodfrom when the step of decreasing starts decreasing the values of theupper limits on electric power input/output to when the connection unitcompletes switching of connection between the plurality of sub powerstorage devices and the second voltage converter.

With this configuration, charging and discharging limits on electricpower of the main power storage device are temporarily relaxed in aperiod in which electric power cannot be input/output to/from the subpower storage device due to switching of connection of the sub powerstorage device. This can ensure upper limits on electric powerinput/output in the entire power supply system.

More preferably, the electrically powered vehicle further includes aninternal combustion engine configured to be capable of outputting powerto drive the vehicle independently of the motor, and a traveling controlunit. The traveling control unit is configured to start the internalcombustion engine when total required power for the electrically poweredvehicle is greater than a sum of a value of an upper limit on electricpower output from the main power storage device and the value of theupper limit on electric power output from the selected sub power storagedevice.

With this configuration, by appropriately setting the values of theupper limits on electric power input/output at the time of switchingconnection of the sub power storage device, the power supply system canbe prevented from being requested to excessively charge/discharge. Inaddition, by temporarily relaxing charging and discharging limits forthe main power storage device, the internal combustion engine can beprevented from being restarted at the time of switching connection ofthe sub power storage device.

Effects of the Invention

According to the present invention, in a power supply system configuredto include a main power storage device and a plurality of sub powerstorage devices, and a voltage converter (a converter) shared by theplurality of sub power storage devices, a connection switching processfor changing a sub power storage device to be used can be performedappropriately.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a main configuration of an electricallypowered vehicle equipped with a power supply system in accordance withan embodiment of the present invention.

FIG. 2 is a circuit diagram showing a detailed configuration of eachinverter shown in FIG. 1.

FIG. 3 is a circuit diagram showing a detailed configuration of eachconverter shown in FIG. 1.

FIG. 4 is a functional block diagram for illustrating how traveling ofthe electrically powered vehicle is controlled.

FIG. 5 is a flowchart showing a schematic procedure of a processperformed to switch connection of a selected sub power storage device inthe power supply system for the electrically powered vehicle accordingto the embodiment of the present invention.

FIG. 6 is a flowchart for illustrating in detail a process performed todetermine whether the sub power storage device should be switched, asshown in FIG. 5.

FIG. 7 is a flowchart for illustrating in detail a pre-switching voltagestep-up process shown in FIG. 5.

FIG. 8 is a flowchart for illustrating in detail an electric power limitmodification process shown in FIG. 5.

FIG. 9 is a flowchart for illustrating in detail a connection switchingprocess shown in FIG. 5.

FIG. 10 is a flowchart for illustrating in detail a return process shownin FIG. 5.

FIG. 11 is a waveform diagram of an operation performed in the processfor switching the selected sub power storage device in the power supplysystem for the electrically powered vehicle according to the embodimentof the present invention.

FIG. 12 is a functional block diagram for illustrating a functionalportion for the process for switching the selected sub power storagedevice, in a configuration controlling the power supply system of theembodiment of the present invention.

DESCRIPTION OF THE REFERENCE SIGNS

1: electrically powered vehicle, 2: wheel, 3: power split device, 4:engine, 6: battery charging converter (external charging), 8: externalpower supply, 9A, 9B1, 9B2: current sensor, 10A, 10B1, 10B2, 13, 21A,21B: voltage sensor, 11A, 11B1, 11B2: temperature sensor, 12A: converter(dedicated to main power storage device), 12B: converter (shared by subpower storage devices), 14, 22: inverter, 15-17: each phase arm (U, V,W), 24, 25: current sensor, 30: control device, 39A: connection unit(for main power storage device), 39B: connection unit (for sub powerstorage device), 100: switching determination unit, 110: step-up-voltageinstruction unit, 120: electric power limiter unit (for main powerstorage device), 130: electric power limiter unit (for sub power storagedevice), 140: connection switching control unit, 200: converter controlunit, 250: traveling control unit, 260: total power calculation unit,270, 280: inverter control unit, BA: battery (main power storagedevice), BB: selected sub power storage device, BB1, BB2: battery (subpower storage device), C1, C2, CH: smoothing capacitor, CMBT:step-up-voltage command signal, CONT1 to CONT7: relay control signal, D1to D8: diode, FBT: flag (stepping up voltage completed), IA, IB1, IB2:input/output current (battery), ID: variable (status of switchingprocess), IGON: start signal, L1: reactor, MCRT1, MCRT2: motor currentvalue, MG1, MG2: motor generator, PL1A, PL1B: power supply line, PL2:electric power feeding line, Pttl: total required power, PWMI, PWMI1,PWMI2, PWMC, PWMC1, PWMC2: control signal (for inverter), PWU, PWUA,PWDA, PWD, PWDA, PWDB: control signal (for converter), Q1 to Q8: IGBTdevice, R: limiting resistor, SL1, SL2: ground line, SMR1 to SMR3:system main relay, SR1, SR1G, SR2, SR2G: relay, TA, TBB1, TBB2: batterytemperature (battery), Tqcom1, Tqcom2: torque command value, UL, VL, WL:line (three phase), V1: predetermined voltage, VBA, VBB1, VBB2: voltage(battery output voltage), VLA, VLB, VH: voltage, VHref: voltage commandvalue (VH), Win: upper limit on electric power input, Win(M): upperlimit on electric power input (to main power storage device), Win(S):upper limit on electric power input (to selected sub power storagedevice), Wout: upper limit on electric power output, Wout(M): upperlimit on electric power output (from main power storage device),Wout(S): upper limit on electric power output (from selected sub powerstorage device).

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter reference will be made to the drawings to more specificallydescribe the present invention in embodiments. In the followingdescription, identical or equivalent components are denoted by identicalreference characters and will in principle not be described repeatedly.

FIG. 1 is a diagram showing a main configuration of an electricallypowered vehicle equipped with a power supply system in accordance withan embodiment of the present invention.

With reference to FIG. 1, an electrically powered vehicle 1 includespower storage devices implemented as batteries BA, BB1, BB2, connectionunits 39A, 39B, converters 12A, 12B, smoothing capacitors C1, C2, CH,voltage sensors 10A, 10B1, 10B2, 13, 21A, 21B, temperature sensors 11A,11B1, 11B2, current sensors 9A, 9B1, 9B2, an electric power feeding linePL2, inverters 14, 22, motor generators MG1, MG2, a wheel 2, a powersplit device 3, an engine 4, and a control device 30.

A power supply system for the electrically powered vehicle shown in thepresent embodiment includes a main power storage device implemented asbattery BA, electric power feeding line PL2 supplying electric power toinverter 14 driving motor generator MG2, converter 12A provided betweenthe main power storage device (BA) and electric power feeding line PL2to serve as a voltage converter converting voltage bidirectionally,batteries BB1, BB2 implementing a plurality of sub power storage devicesprovided in a manner parallel to each other, and converter 12B providedbetween the plurality of sub power storage devices (BB1, BB2) andelectric power feeding line PL2 to serve as a voltage converterconverting voltage bidirectionally. The voltage converter (12B) isconnected selectively to one of the plurality of sub power storagedevices (BB1, BB2) to convert voltage between the connected sub powerstorage device and electric power feeding line PL2 bidirectionally.

A sub power storage device (one of BB1 and BB2) and the main powerstorage device (BA) have their storable capacity set so that, forexample, when they are concurrently used, they can output maximum powertolerated for an electric load (22 and MG2) connected to the electricpower feeding line. This allows the vehicle without using the engine,i.e., traveling as an EV (Electric Vehicle), to travel with maximumpower. If the sub power storage device's state of power storage isdecreased, the sub power storage device can be exchanged to cause thevehicle to further travel, and if the sub power storage device'selectric power has completely been consumed, then, in addition to themain power storage device, the engine can be used to allow the vehicleto travel with maximum power without using the sub power storage device.

Furthermore, such a configuration allows converter 12B to be shared bythe plurality of sub power storage devices. This can eliminate thenecessity of increasing the number of converters to be equal to that ofpower storage devices. For further increased EV travelable distance, anadditional battery can be introduced in parallel with batteries BB1,BB2.

Preferably, the main power storage device and the sub power storagedevices mounted in this electrically powered vehicle are externallychargeable. For this purpose, electrically powered vehicle 1 furtherincludes a battery charging device (a charging converter) 6 forconnection to an external power supply 8 which is for example acommercial power supply of AC 100V. Battery charging device 6 convertsalternate current into direct current and also adjusts voltage to supplyelectric power charged to a battery. Note that external charging may beachieved by the above described configuration and in addition a systemconnecting a neutral point of a stator coil of motor generator MG1, MG2to an alternate current power supply or a system causing converters 12A,12B to together function as an AC/DC conversion device.

Smoothing capacitor C1 is connected between a power supply line PL1A anda ground line SL2. Voltage sensor 21A detects a voltage VLA across endsof smoothing capacitor C1 and outputs it to control device 30. Converter12A can step up the voltage across terminals of smoothing capacitor C1and supply it to electric power feeding line PL2.

Smoothing capacitor C2 is connected between a power supply line PL1B andground line SL2. Voltage sensor 21B detects a voltage VLB across ends ofsmoothing capacitor C2 and outputs it to control device 30. Converter12B can step up the voltage across terminals of smoothing capacitor C2and supply it to electric power feeding line PL2.

Smoothing capacitor CH smoothes the voltage stepped up by converter 12A,12B. Voltage sensor 13 senses a voltage VH across terminals of smoothingcapacitor CH and outputs it to control device 30.

Alternatively, in an opposite direction, converters 12A, 12B can stepdown voltage VH across terminals smoothed by smoothing capacitor CH andsupply it to power supply lines PL1A, PL1B.

Inverter 14 receives direct current voltage from converter 12B and/or12A, converts it into three-phase alternate current voltage, and outputsit to motor generator MG1. Inverter 22 receives direct current voltagefrom converter 12B and/or 12A, converts it into three-phase alternatecurrent voltage, and outputs it to motor generator MG2.

Power split device 3 is a mechanism coupled to engine 4 and motorgenerators MG1, MG2 to distribute motive power therebetween. The powersplit device can for example be a planetary gear mechanism having threeshafts of rotation of a sun gear, a planetary carrier, and a ring gear.In the planetary gear mechanism, when two of the three shafts ofrotation have their rotation determined, that of the other one shaft ofrotation is compulsively determined. These three shafts of rotation areconnected to engine 4 and motor generators MG1, MG2 at their respectiveshafts of rotation, respectively. Motor generator MG2 has its shaft ofrotation coupled to wheel 2 by a reduction gear, a differential gear orthe like (not shown). Furthermore, power split device 3 may further havea speed reducer incorporated therein for the shaft of rotation of motorgenerator MG2.

Connection unit 39A includes a system main relay SMR2 connected betweenthe positive electrode of battery BA and power supply line PL1A, asystem main relay SMR1 and a limiting resistor R connected in series andconnected in parallel with system main relay SMR2, and a system mainrelay SMR3 connected between the negative electrode of battery BA (aground line SL1) and a node N2.

System main relays SMR1 to SMR3 have their conduction/non-conductionstates controlled (or are turned on/off) by relay control signals CONT1to CONT3, respectively, issued from control device 30.

Voltage sensor 10A measures a voltage VA across terminals of battery BA.Furthermore, temperature sensor 11A measures a temperature TA of batteryBA, and current sensor 9A measures a current IA input/output to/frombattery BA. These sensors' measurements are output to control device 30.Based on these measurements, control device 30 monitors a state ofbattery BA represented by the state of charge (SOC).

Connection unit 39B is provided between power supply line PL1B andground line SL2, and batteries BB1, BB2. Connection unit 39B includes arelay SR1 connected between the positive electrode of battery BB1 andpower supply line PL1B, a relay SR1G connected between the negativeelectrode of battery BB1 and ground line SL2, a relay SR2 connectedbetween the positive electrode of battery BB2 and power supply linePL1B, and a relay SR2G connected between the negative electrode ofbattery BB2 and ground line SL2.

Relays SR1, SR2 have their conduction/non-conduction states controlled(or are turned on/off) by relay control signals CONT4, CONT5,respectively, issued from control device 30. Relays SR1G, SR2G havetheir conduction/non-conduction states controlled (or are turned on/off)by relay control signals CONT6, CONT7, respectively, issued from controldevice 30. Ground line SL2 extends through converters 12A, 12B towardinverters 14 and 22, as will be described later.

Voltage sensors 10B1 and 10B2 measure voltages VBB1 and VBB2 acrossterminals of batteries BB1 and BB2, respectively. Temperature sensors11B1 and 11B2 measure temperatures TBB1 and TBB2 of batteries BB1 andBB2, respectively. Current sensors 9B1 and 9B2 measure currents IB1 andIB2 input/output to/from batteries BB1 and BB2, respectively. Thesesensors' measurements are output to control device 30. Based on thesemeasurements, control device 30 monitors states of batteries BB1, BB2represented by the states of charge (SOC).

Battery BA, BB1, BB2 can for example be a lead-acid battery, a nickelmetal hydride battery, a lithium ion battery or a similar secondarybattery, an electric double layer capacitor or a similar capacitor oflarge capacity, or the like.

Inverter 14 is connected to electric power feeding line PL2 and groundline SL2. Inverter 14 receives a voltage stepped up from converter 12Aand/or converter 12B, and drives motor generator MG1 for example tostart engine 4. Furthermore, inverter 14 returns to converters 12A and12B the electric power generated by motor generator MG1 by motive powertransmitted from engine 4. At this time, converters 12A and 12B arecontrolled by control device 30 to operate as step-down converters.

Current sensor 24 detects a current that flows to motor generator MG1 asa motor current value MCRT1, and outputs motor current value MCRT1 tocontrol device 30.

Inverter 22 is connected to electric power feeding line PL2 and groundline SL2 in a manner parallel with inverter 14. Inverter 22 receivesdirect current voltage from converters 12A and 12B, converts it intothree-phase alternate current voltage, and outputs it to motor generatorMG2 driving wheel 2. Furthermore, inverter 22 returns to converters 12Aand 12B the electric power generated by motor generator MG2 as thevehicle is regeneratively braked. At this time, converters 12A and 12Bare controlled by control device 30 to operate as step-down converters.

Current sensor 25 detects a current that flows to motor generator MG2 asa motor current value MCRT2, and outputs motor current value MCRT2 tocontrol device 30.

Control device 30 is constituted of an electronic control unit (ECU)having a central processing unit (CPU) and a memory (not shown)incorporated therein, and in accordance with a map and a program storedin the memory, uses each sensor's measurement to perform operationprocessing. Note that control device 30 may have a portion configured toallow an electronic circuit or similar hardware to perform predeterminedarithmetic and logical operation processing.

More specifically, control device 30 receives torque command values androtation speeds of motor generators MG1, MG2, values of voltages VBA,VBB1, VBB2, VLA, VLB, VH, motor current values MCRT1, MCRT2, and a startsignal IGON. Then, control device 30 outputs a control signal PWUBinstructing converter 12B to step up voltage, a control signal PWDBinstructing converter 12B to step down voltage, and a shutdown signalinstructing converter 12B to prohibit operation.

Furthermore, control device 30 outputs a control signal PWMI1instructing inverter 14 to convert direct current voltage output fromconverters 12A, 12B into alternate current voltage for driving motorgenerator MG1, and a control signal PWMC1 instructing inverter 14 toconvert alternate current voltage generated by motor generator MG1 intodirect current voltage and return it toward converters 12A, 12B forregeneration.

Similarly, control device 30 outputs a control signal PWMI2 instructinginverter 22 to convert direct current voltage into alternate currentvoltage for driving motor generator MG2, and a control signal PWMC2instructing inverter 22 to convert alternate current voltage generatedby motor generator MG2 into direct current voltage and return it towardconverters 12A, 12B for regeneration.

FIG. 2 is a circuit diagram showing a detailed configuration ofinverters 14 and 22 shown in FIG. 1.

With reference to FIG. 2, inverter 14 includes a U phase arm 15, a Vphase arm 16, and a W phase arm 17. U phase arm 15, V phase arm 16, andW phase arm 17 are connected between electric power feeding line PL2 andground line SL2 in parallel.

U phase arm 15 includes insulated gate bipolar transistor (IGBT) devicesQ3, Q4 connected in series between electric power feeding line PL2 andground line SL2, IGBT devices Q3, Q4, and their respective anti-paralleldiodes D3, D4. Diode D3 has its cathode connected to IGBT device Q3 atthe collector, and its anode connected to IGBT device Q3 at the emitter.Diode D4 has its cathode connected to IGBT device Q4 at the collector,and its anode connected to IGBT device Q4 at the emitter.

V phase arm 16 includes IGBT devices Q5, Q6 connected in series betweenelectric power feeding line PL2 and ground line SL2, and theirrespective anti-parallel diodes D5, D6. IGBT devices Q5, Q6 andanti-parallel diodes D5, D6 are connected similarly as in U phase arm15.

W phase arm 17 includes IGBT devices Q7, Q8 connected in series betweenelectric power feeding line PL2 and ground line SL2, and theirrespective anti-parallel diodes D7, D8. IGBT devices Q7, Q8 andanti-parallel diodes D7, D8 are also connected similarly as in U phasearm 15.

Note that, in the present embodiment, an IGBT device is indicated as arepresentative example of a power semiconductor switching elementcontrollable to be turned on/off. In other words, it is also replaceablewith a bipolar transistor, a field effect transistor or a similar powersemiconductor switching element.

Each phase arm has an intermediate point connected to motor generatorMG1 at each phase coil at each phase end. In other words, motorgenerator MG1 is a three-phase permanent magnet synchronous motor, andthe three U, V, W phase coils each have one end connected together to anintermediate point. The U phase coil has the other end connected to aline UL drawn from a connection node of IGBT devices Q3, Q4. The V phasecoil has the other end connected to a line VL drawn from a connectionnode of IGBT devices Q5, Q6. The W phase coil has the other endconnected to a line WL drawn from a connection node of IGBT devices Q7,Q8.

Inverter 22 shown in FIG. 1 is different in that it is connected tomotor generator MG2. However, its internal circuit configuration issimilar to that of inverter 14, and accordingly it will not be describedrepeatedly in detail. Furthermore, FIG. 2 shows an inverter receivingcontrol signals PWMI, PWMC, however, this is to avoid complexity. Asshown in FIG. 1, different control signals PWMI1, PWMC1 and controlsignals PWMI2, PWMC2 are input to inverters 14, 22, respectively.

FIG. 3 is a circuit diagram showing a detailed configuration ofconverters 12A and 12B shown in FIG. 1.

With reference to FIG. 3, converter 12A includes a reactor L1 having oneend connected to power supply line PL1A, IGBT devices Q1, Q2 connectedin series between electric power feeding line PL2 and ground line SL2,and their respective anti-parallel diodes D1, D2.

Reactor L1 has the other end connected to IGBT device Q1 at the emitterand to IGBT device Q2 at the collector. Diode D1 has its cathodeconnected to IGBT device Q1 at the collector and its anode connected toIGBT device Q1 at the emitter. Diode D2 has its cathode connected toIGBT device Q2 at the collector, and its anode connected to IGBT deviceQ2 at the emitter.

Converter 12B shown in FIG. 1 is different from converter 12A in thatthe former is not connected to power supply line PL1A and instead topower supply line PL1B. However, its internal circuit configuration issimilar to that of converter 12A, and accordingly it will not bedescribed repeatedly in detail. Furthermore, FIG. 3 shows a converterreceiving control signals PWU, PWD, however, this is to avoidcomplexity. As shown in FIG. 1, different control signals PWUA, PWDA andcontrol signals PWUB, PWDB are input to inverters 14, 22, respectively.

In the power supply system for electrically powered vehicle 1, batteryBA (the main power storage device) and a sub power storage deviceselected from batteries BB1, BB2 (hereinafter also referred to as a“selected sub power storage device BB”), and motor generators MG1, MG2supply and receive electric power therebetween.

Control device 30 receives values detected by voltage sensor 10A,temperature sensor 11A, and current sensor 9A, and in accordancetherewith sets an SOC(M) indicating the main power storage device'sresidual capacity, an upper limit on electric power input Win(M)indicating an upper limit value of electric power charged thereto, andan upper limit on electric power output Wout(M) indicating an upperlimit value of electric power discharged therefrom.

Furthermore, control device 30 receives values detected by voltagesensors 10B1, 10B2, temperature sensors 11B1, 11B2 and current sensors9B1, 9B2, and in accordance therewith sets an SOC(B) of selected subpower storage device BB and upper limits on electric power input andoutput Win(S), Wout(S) thereto and therefrom, respectively.

Generally, an SOC is indicated by a ratio (%) of each battery's currentcharged amount to its fully charged state. Furthermore, Win, Wout areindicated as such an upper limit value of electric power that, when thatelectric power is discharged for a predetermined period of time (e.g.,for approximately 10 seconds), the battery of interest (BA, BB1, BB2) isnot overcharged/overdischarged.

FIG. 4 is a functional block diagram for illustrating how control device30 controls traveling of electrically powered vehicle 1, morespecifically, a configuration of power distribution control betweenengine 4 and motor generators MG1, MG2. FIG. 4 shows function blocks,which are implemented by control device 30 executing a previouslystored, predetermined program and/or by processing of an operation byelectronic circuitry (hardware) in control device 30.

With reference to FIG. 4, a total power calculation unit 260 calculatestotal power Pttl required for the entirety of electrically poweredvehicle 1 from a vehicular speed and an operation of a pedal (anaccelerator pedal). Note that total required power Pttl may also includepower required (i.e., the engine's output), depending on the vehicle'scondition, for generating electric power by motor generator MG1 tocharge a battery.

A traveling control unit 250 receives upper limits on electric powerinput/output Win(M), Wout(M) to/from main power storage device BA, upperlimits on electric power input/output Win(S), Wout(S) to/from selectedsub power storage device BB, total required power Pttl from total powercalculation unit 260, and a regenerative brake request made when a brakepedal is operated. Traveling control unit 250 generates a control motorcommand, or torque command values Tqcom1 and Tqcom2, to allow motorgenerators MG1, MG2 in total to receive/output electric power within acharging limit (Win(M)+Win(S)) and a discharging limit (Wout(M)+Wout(S))in total for main power storage device BA and selected sub power storagedevice BB. Furthermore, to ensure total required power Pttl, it isassigned between power provided by motor generator MG2 to drive thevehicle and that provided by engine 4 to do so. In particular,externally charged battery's electric power is maximally utilized torestrict engine 4 from operation, or the power provided by engine 4 todrive the vehicle is set to correspond to a range allowing engine 4 tobe highly efficiently operable, to control the vehicle to travel toachieve high fuel-efficiency.

An inverter control unit 270 receives torque command value Tqcom1 andmotor current value MCRT1 of motor generator MG1, and therefromgenerates control signals PWMI1, PWMC1 for inverter 14. Similarly, aninverter control unit 280 receives torque command value Tqcom2 and motorcurrent value MCRT2 of motor generator MG2, and therefrom generatescontrol signals PWMI2, PWMC2 for inverter 22. Furthermore, travelingcontrol unit 250 generates a control engine command in response to avalue requested of power provided by the engine to drive the vehicle, asset. Furthermore, a control device (an engine ECU) (not shown) controlsthe operation of engine 4 in accordance with the control engine command.

In a travel mode in which the vehicle travels actively using a battery'selectric power (i.e., in an EV mode), when total required power Pttl isequal to or smaller than the batteries' total upper limit on electricpower output Wout(M)+Wout(S), control device 30 does not operate engine4, and motor generator MG2 alone provides power to drive the vehicle totravel. When total required power Pttl exceeds Wout(M)+Wout(S), engine 4is started.

In contrast, in a travel mode in which the EV mode is not selected(i.e., in an HV mode), control device 30 controls distribution ofdriving power between engine 4 and motor generator MG2 to maintain thebatteries' SOC at a predetermined target value. In other words,traveling control under which travel with engine 4 is more actuatablethan in the EV mode is carried out.

In the EV mode, charging and discharging are controlled topreferentially use the electric power of selected sub power storagedevice BB rather than that of main power storage device BA. As such,when the vehicle is traveling and currently used, selected sub powerstorage device BB is decreased in SOC, selected sub power storage deviceBB needs to be switched. For example, if battery BB1 is set as selectedsub power storage device BB in starting the vehicle, necessity willarise to subsequently disconnect battery BB1 from converter 12B andconnect battery BB2 as a newly selected sub power storage device BB toconverter 12B, i.e., to perform a connection switching process.

On this occasion, battery BB2 newly set as selected sub power storagedevice BB generally has an output voltage higher than that of batteryBB1 that has been used so far. As a result, connection of a newhigh-voltage battery may cause occurrence of an unintended short-circuitpath, which may pose a problem in protection of equipment and the like.Therefore, in the process for switching connection of the sub powerstorage device, full attention should be paid to prevent occurrence of ashort-circuit path. Further, since supply and recovery of electric powerby selected sub power storage device BB cannot be performed during aperiod of the connection switching process described above, it isrequired to limit charging and discharging to prevent occurrence ofovercharging and overdischarging in the entire power supply systemduring that period.

Hereinafter, the connection switching process for the sub power storagedevice with attention being paid to such disadvantages will bedescribed.

FIG. 5 is a flowchart showing a schematic procedure of a processperformed to switch a selected sub power storage device in the powersupply system for the electrically powered vehicle according to theembodiment of the present invention. Furthermore, FIGS. 6 to 10 areflowcharts for specifically illustrating steps S100, S200, S300, S400,and S500 in FIG. 5.

Control device 30 can execute a previously stored, predetermined programperiodically as predetermined, to repeatedly perform a control processprocedure in accordance with the flowcharts shown in FIGS. 5 to 10,periodically as predetermined. Thereby, the connection switching processfor the sub power storage device in the power supply system for theelectrically powered vehicle according to the embodiment of the presentinvention can be implemented.

With reference to FIG. 5, in step S100, control device 30 performs aprocess for determining whether a selected sub power storage deviceshould be switched. If control device 30 determines that it is necessaryto switch the selected sub power storage device, the following stepsS200 to S500 are performed. If control device 30 determines in step S100that it is not necessary to switch the selected sub power storagedevice, steps S200 to S500 are substantially not performed.

In step S200, control device 30 performs a pre-switching voltage step-upprocess, and in step S300, control device 30 performs an electric powerlimit modification process so that a request is not generated to thepower supply system to excessively charge/discharge while connection ofthe sub power storage device is being switched. In step S400, controldevice 30 performs a connection switching process for actually switchingconnection between selected sub power storage device BB and converter12B, and after the process is completed, control device 30 performs instep S500 a return process to start supplying electric power by newlyselected sub power storage device BB.

FIG. 6 is a flowchart for illustrating in detail the process performedto determine whether the selected sub power storage device should beswitched (S100), as shown in FIG. 5.

As will be described hereinafter, a variable ID is introduced toindicate the progress (i.e., a status) of the connection switchingprocess. Variable ID is set to any of −1 and 0 to 4. ID=0 indicates astate in which no request for switching a sub power storage device isgenerated. In other words, when ID=0, currently selected sub powerstorage device BB supplies electric power, while whether selected subpower storage device BB should be switched or not is determinedperiodically as predetermined. If there is no sub power storage devicethat can newly be used due to failure in equipment or consumed electricpower in the battery, it is assumed that ID is set to −1 (ID=−1).

With reference to FIG. 6, in step S105, control device 30 determineswhether ID=0 or not. If ID=0 (YES in S105), in step S110, control device30 determines whether or not the selected sub power storage deviceshould be switched. Determination in step S110 is basically made basedon the SOC of the currently selected sub power storage device. That is,if the SOC of the sub power storage device in use becomes lower than aprescribed reference value, it is determined that the selected sub powerstorage device should be switched.

In step S150, control device 30 confirms a result of determination instep S110 as to whether switching is necessary or not. When it isdetermined that switching is necessary (YES in step S150), controldevice 30 designates selected sub power storage device BB to be newlyused in step S160. In a case where two batteries BB1, BB2 are mounted asthe sub power storage devices as shown in FIG. 1, newly selected subpower storage device BB is automatically determined without the need toperform the process in step S160. However, in a case where three or moresub power storage devices BB1 to BBn (where n is an integer equal to orgreater than 3) are mounted in the configuration of FIG. 1, a new subpower storage device to be used subsequently is designated based on therespective SOCs of the sub power storage devices that are not currentlyused, and the like. Then, control device 30 sets ID=1 in order toproceed with the connection switching process. Namely, ID=1 indicates astate in which a request for switching selected sub power storage deviceBB is generated and the switching process is started.

On the other hand, if control device 30 determines in step S110 that itis not necessary to switch the selected sub power storage device (NO inS150), control device 30 maintains ID=0 in step S170. If ID≧1 is oncesatisfied and the switching process has been started, or if there is nosub power storage device that can newly be used and ID=−1 is set (NO inS105), the processes in steps S110 to S180 are skipped.

FIG. 7 is a flowchart for illustrating in detail the pre-switchingvoltage step-up process (S200) shown in FIG. 5.

With reference to FIG. 7, in the pre-switching voltage step-up process,control device 30 confirms whether ID=1 or not in step S205. If ID=1, aswitching request for switching selected sub power storage device BB ismade and the switching process is started (YES in S205), control device30 generates in step S210 a command to converter 12A to step up voltageVH on electric power feeding line PL2 to a predetermined voltage V1. Inresponse to the step-up voltage command, a voltage command value VHreffor electric power feeding line PL2 is set to be equal to V1, andcontrol signal PWUA for converter 12A is generated to implement thisvoltage command value.

Note that predetermined voltage V1 is set to be higher than any higherone of respective output voltages of main power storage device BA andselected sub power storage device BB to be newly connected (for example,BB2). For example, predetermined voltage V1 set at an upper limitcontrol voltage VHmax that can be stepped up by converter 12A can ensurethat voltage VH when a step-up voltage command is issued is higher thanboth of the output voltages of main power storage device BA and selectedsub power storage device BB after switching. Alternatively, in view ofreducing a loss caused at converter 12A, predetermined voltage V1 may bedetermined, as occasion demands, to have a margin, depending on voltagesoutput from main power storage device BA and selected sub power storagedevice BB after switching at that time.

If the step-up voltage command is generated in step S210, control device30 determines in step S220 whether or not voltage VH has reachedpredetermined voltage V1, based on a value detected by voltage sensor13. Determination as YES is made in step S220, for example, when VH≧V1continues for a predetermined period of time.

Once voltage VH has reached predetermined voltage V1 (YES in S220),control device 30 furthers the ID from 1 to 2. Until voltage VH reachesV1 (NO in S220), ID=1 is maintained. In other words, ID=2 indicates astate in which the pre-switching voltage step-up process ends and theswitching process can be furthered. If ID≠1 (NO in S205), the processesin subsequent steps S210 to S230 are skipped.

Thus, when the pre-switching voltage step-up process (step S200) ends,control device 30 performs the electric power limit modification processas shown in FIG. 8.

FIG. 8 is a flowchart for illustrating in detail the electric powerlimit modification process (S300) shown in FIG. 5.

With reference to FIG. 8, in the electric power limit modificationprocess, control device 30 initially determines whether or not ID=2 instep S305. If ID=2 is not satisfied (NO in S305), processes insubsequent steps S310 to S340 are skipped.

If ID=2 (YES in S305), control device 30 starts temporary relaxation ofcharging and discharging limits for main power storage device BA in stepS310. Specifically, absolute values of upper limits on electric powerinput/output Win(M), Wout(M) to/from main power storage device BA aretemporarily increased.

Further, in step S320, control device 30 gradually decreases absolutevalues of upper limits on electric power input/output Win(S), Wout(S)to/from selected sub power storage device BB. For example, Wout(S),Win(S) are decreased gradually toward 0 at a predetermined fixed rate.

In step S330, control device 30 determines whether or not Wout(S),Win(S) have reached 0. Until Wout(S)=Win(S)=0, step S320 is repeated tocontinuously decrease Wout(S) and Win(S).

Once Wout(S) and Win(S) have reached 0 (YES in S330), control device 30furthers the ID from 2 to 3 in step S340. In other words, ID=3 indicatesa state in which the pre-switching voltage step-up process and theelectric power limit modification process have ended and switching ofconnection between sub power storage devices BB1, BB2 and converter 12Bcan be started.

When the electric power limit modification process shown in FIG. 8 ends,control device 30 performs the connection switching process for the subpower storage device in step S400.

FIG. 9 is a flowchart for illustrating in detail the connectionswitching process for the sub power storage device (S400), as shown inFIG. 5.

With reference to FIG. 9, in the connection switching process for thesub power storage device, control device 30 initially determines whetheror not ID=3 in step S405. If ID≠3 (NO in S405), processes in subsequentsteps S410 to S450 are skipped.

If ID=3 (YES in S405), control device 30 stops converter 12B to preparefor switching connection of the sub power storage device in step S410.More specifically, in converter 12B, IGBT devices Q1, Q2 are forced tobe turned off in response to a shutdown command, and in that condition,control device 30 generates in step S420 a relay control signal foractually switching connection of the sub power storage device. Forexample, in order to disconnect battery BB1 from converter 12B andconnect battery BB2 with converter 12B, relay control signals CONT4,CONT6 are generated to turn off relays SR1, SR1G, and relay controlsignals CONT5, CONT7 are generated to turn on SR2, SR2G.

Furthermore, in step S430, control device 30 determines whether or notrelay connection switching as instructed in step S420 has beencompleted. When the connection switching has been completed (YES inS430), control device 30 restarts converter 12B to start a switchingoperation in step S440, and furthers the ID from 3 to 4 in step S450.

In other words, ID=4 indicates a state in which switching of connectionbetween the sub power storage devices and converter 12B by means of therelays has been completed.

When the connection switching process in step S400 ends, control device30 performs the return process in step S500.

FIG. 10 is a flowchart for illustrating in detail the return process(S500) shown in FIG. 5.

With reference to FIG. 10, in the return process, control device 30initially determines whether or not ID=4 in step S505. If ID≠4 (NO inS505), processes in subsequent steps S510 to S570 are skipped.

If ID=4 (YES in S505), in step S510, control device 30 ends thetemporary relaxation of charging and discharging limits for main powerstorage device BA started in step S310 (FIG. 7). Thereby, Wout(M) andWin(M) basically return to values before the start of the switchingprocess for selected power storage device BB.

Further, control device 30 gradually increases upper limits on electricpower input/output Win(S), Wout(S) to/from selected sub power storagedevice BB decreased to 0 in the electric power limit modificationprocess (step S300), to values of Win, Wout to/from a newly selected subpower storage device (for example, battery BB2).

Then, in step S530, control device 30 confirms whether or not upperlimits on electric power input/output Win(S), Wout(S) have returned tothe values of Win, Wout to/from newly selected sub power storage deviceBB. During a period until return is completed (NO in S530), step S520 isrepeatedly performed to gradually increase upper limits on electricpower input/output Win(S), Wout(S) at a fixed rate.

When return of upper limits on electric power input/output Win(S),Wout(S) is completed (YES in S530), control device 30 returns the IDback to 0 in step S540. Thereby, a state in which normal supply andrecovery of electric power by main power storage device BA and newlyselected sub power storage device BB can be performed is reproduced inthe power supply system.

Further, the process proceeds to step S550 and control device 30 turnsoff the step-up voltage command generated in step S210 (FIG. 6). Thus,the voltage command value for electric power feeding line PL2 is alsoset to an ordinary value set in accordance with the states of motorgenerators MG1, MG2.

After completion of a series of switching processes, control device 30may further determine whether or not there is a possibility that furtherswitching of the selected sub power storage device is performed duringtraveling of the vehicle in step S560. If there is no possibility offurther switching, control device 30 sets ID=−1 in step S570. If ID=−1is set, steps S100 to S500 in FIG. 4 are substantially not performed,and thus the switching process for the selected sub power storage deviceis not started until the vehicle stops operation.

On the other hand, if there is a possibility of further switching,control device 30 skips step S570 and maintains ID=0. As a result, theswitching determination process in step S100 is performed periodicallyas predetermined, and thereby the switching process for the selected subpower storage device is restarted as necessary.

Note that, in the exemplary configuration of FIG. 1 in which only twosub power storage devices are mounted, it is possible to omit theprocess in step S560, that is, always set ID=−1 once the switchingprocess for the selected sub power storage device is completed, therebylimiting the number of the switching process for the selected sub powerstorage device performed during driving of the vehicle to only one.

Alternatively, in a power supply system equipped with three or more subpower storage devices or a power supply system having a configurationsuch that a sub power storage device not in use can be charged duringdriving of a vehicle, the power supply system can be configured suchthat a second or later switching process for a selected sub powerstorage device can be performed by maintaining ID=0 depending on asituation.

FIG. 11 shows an operation waveform in the process for switching theselected sub power storage device in the power supply system for theelectrically powered vehicle according to the embodiment of the presentinvention described with reference to FIGS. 5 to 10.

With reference to FIG. 11, during a period until time t1 when ID=0, theswitching determination process is performed periodically aspredetermined, based on the SOC of the currently selected sub powerstorage device (e.g., battery BB1).

At time t1, in response to a decrease in the SOC of battery BB1, theswitching determination process (step S100) is performed to issue aswitching request to switch selected sub power storage device BB, andID=1 is set to start the switching process.

Thus, the pre-switching voltage step-up process (step S200) is performedand converter 12A increases voltage VH on electric power feeding linePL2 toward predetermined voltage V1. The process for stepping up voltageon electric power feeding line PL2 is completed at time t2, andaccordingly, the ID is changed from 1 to 2.

When ID=2 is set, the electric power limit modification process (S300)is performed to temporarily relax charging and discharging for mainpower storage device BA. Specifically, temporary increase in theabsolute values of upper limits on electric power input/output Win(M),Wout(M) is started. Further, upper limits on electric power input/outputWin(S), Wout(S) to/from selected sub power storage device BB aredecreased toward 0 gradually at a fixed rate. Note that, during thisperiod, converter 12B is controlled to stop charging/discharging of thecurrently selected sub power storage device (battery BB1).Alternatively, converter 12B may be shut down from time t1.

At time t3, upper limits on electric power input/output Win(S), Wout(S)to/from selected sub power storage device BB are decreased to 0, and inresponse, the ID is changed from 2 to 3. Once ID=3 is set, theconnection switching process for the sub power storage device isstarted. More specifically, with converter 12A being shut down, relaysSR1, SR1G are turned off, and thereafter relays SR2, SR2G are turned on.Then, when the relay connection switching process is completed andbattery BB2 as a newly selected sub power storage device is connected toconverter 12B, converter 12B is restarted. By completing this connectionswitching process, the ID is changed from 3 to 4 at time t4.

When ID=4 is set, upper limits on electric power input/output Win(S),Wout(S) to/from selected sub power storage device BB are graduallyincreased at a fixed rate, and thereby battery BB2 is started to be usedas a newly selected sub power storage device. Accordingly, the temporaryrelaxation of charging and discharging limits for main power storagedevice BA ends, and Wout(M), Win(M) are basically caused to return tovalues before time t2.

When Win(S), Wout(S) to/from selected sub power storage device BB returnto original values corresponding to Wout, Win from/to battery BB2 attime t5, the ID returns to 0. Then, the process for stepping up voltageon electric power feeding line PL2 is also stopped.

Thereby, a series of switching processes for the selected sub powerstorage device are completed, and a state in which normal supply andrecovery of electric power using selected sub power storage device BB(battery BB2) can be performed is reproduced.

At time t5, if it is determined whether there is a possibility that afurther process for switching the sub power storage device is performedduring driving of the vehicle, and ID=−1 is set when there is nopossibility of occurrence of the switching process as described in FIG.10, load subsequently imposed on control device 30 can be alleviated.

Next, a configuration of a functional portion for the process forswitching the selected sub power storage device described in FIGS. 5 to10, which a part of a configuration controlling the power supply systemof the embodiment of the present invention will be described withreference to FIG. 12. FIG. 12 shows function blocks, which areimplemented as control device 30 executing a predetermined program toprovide software processing, or by dedicated electronic circuitry (orhardware processing).

With reference to FIG. 12, a switching determination unit 100 receivesSOC(BB1), SOC(BB2) indicating the states of charge of batteries BB1,BB2, respectively, and determines whether or not the SOC of selected subpower storage device BB currently in use is lower than a predeterminedreference value. When variable ID shared by the function blocks is 0,switching determination unit 100 performs the determination processdescribed above periodically as predetermined, and if it is necessary toswitch the selected sub power storage device, switching determinationunit 100 changes the ID from 0 to 1. Thus, a request for switching theselected sub power storage device is generated. In other words,switching determination unit 100 has a function corresponding to theprocess in step S100 in FIG. 5.

When the request for switching the selected sub power storage device isgenerated and ID=1 is set, a step-up-voltage instruction unit 110outputs a step-up voltage command signal CMBT to a converter controlunit 200 controlling converter 12A.

Converter control unit 200 generates control signals PWUA, PWDA forconverter 12A, based on voltages VH, VLA and voltage command valueVHref, so that voltage VH on electric power feeding line PL2 reachesvoltage command value VHref.

Furthermore, when step-up-voltage instruction unit 110 generates step-upvoltage command signal CMBT, converter control unit 200 sets voltagecommand value VHref=V1 and generates control signal PWUA. If voltagesensor 13 detects voltage VH having reached predetermined voltage V1continuously for at least a predetermined period of time, convertercontrol unit 200 sets a flag FBT indicating that stepping up voltage iscompleted, to ON.

In response to flag FBT set to ON, step-up-voltage instruction unit 110changes the ID to 2, and continues to output step-up voltage commandsignal CMBT until a connection switching control unit 140, which will bedescribed later, completes relay connection switching and accordinglyID=4 is set. In other words, step-up-voltage instruction unit 110 has afunction corresponding to step S200 in FIG. 5 and step S550 in FIG. 10.

An electric power limiter unit 120 sets upper limits on electric powerinput/output Win(S), Wout(S) to/from selected sub power storage deviceBB. Normally, upper limits on electric power input/output Win(S),Wout(S) are set based on selected sub power storage device BB orbattery's SOC (SOC(BB1) or SOC(BB2)), temperature (TBB1 or TBB2), andoutput voltage (VB1 or VB2).

In the process for switching the selected sub power storage device, incontrast, when ID=2 is set, electric power limiter unit 120 decreasesupper limits on electric power input/output Win(S), Wout(S) gradually ata fixed rate toward 0, and when Win(S), Wout(S) have reached 0, electricpower limiter unit 120 changes the ID from 2 to 3. Further, whenconnection switching control unit 140 sets ID=4, electric power limiterunit 120 increases upper limits on electric power input/output Win(S),Wout(S) to values corresponding to Win, Win of newly selected sub powerstorage device BB after switching. Then, when the increasing process iscompleted, electric power limiter unit 120 changes the ID from 4 to 0.

In other words, electric power limiter unit 120 implements the processesin steps S320 to S340 in FIG. 8 and the processes in steps S520 to S540in FIG. 10, and functions of the “first electric power limiter unit” andthe “second electric power limiter unit” of the present invention.

An electric power limiter unit 130 sets upper limits on electric powerinput/output Win(M) and Wout(M) to/from main power storage device BA.Normally, upper limits on electric power input/output Win(M), Wout(M)are set based on main power storage device BA's SOC (BA), temperatureTA, and output voltage VA.

In the switching process for the selected sub power storage device, incontrast, when ID=2 is set, electric power limiter unit 130 temporarilyincreases the absolute values of upper limits on electric powerinput/output Win(M) and Wout(M), and thereby temporarily relaxescharging and discharging limits for main power storage device BA. Then,when connection switching control unit 140 sets ID=4, electric powerlimiter unit 130 causes upper limits on electric power input/outputWin(M) and Wout(M) to return to normal values.

In other words, electric power limiter unit 130 implements the processesin step S310 in FIG. 8 and in step S510 in FIG. 10, and a function ofthe “third electric power limiter unit” of the present invention.

When electric power limiter unit 120 sets ID=3, connection switchingcontrol unit 140 generates a command to shut down converter 12B, andalso generates relay control signals CONT4 to CONT7 to switch connectionbetween converter 12B and sub power storage devices BB1, BB2. Forexample, when selected sub power storage device BB is switched frombattery BB1 to battery BB2, relay control signals CONT4 to CONT7 aregenerated to turn off relays SR1, SR1G and turn on relays SR2, SR2G.Once this relay connection switching process is completed, connectionswitching control unit 140 stops the shutdown command described above torestart converter 12B, and changes the ID from 3 to 4.

Connection switching control unit 140 corresponds to the process in stepS400 in FIG. 5 (the processes in S405 to S450 in FIG. 9).

As described above, according to the power supply system for theelectrically power vehicle in accordance with the present embodiment, inthe process for switching a sub power storage device that is used,voltage on electric power feeding line PL2 is increased and thereafterconnection of the sub power storage device is switched. This ensuresthat connection switching is performed without forming a short-circuitpath originating from a newly used sub power storage device in thesystem. Furthermore, during the switching process for the selected subpower storage device, upper limits on electric power input/outputWin(S), Wout(S) to/from selected sub power storage device BB areappropriately limited, which can prevent the power supply system frombeing requested to excessively charge/discharge. As a result, in a powersupply system configured such that a plurality of sub power storagedevices share a single voltage converter (a converter), a connectionswitching process for a sub power storage device performed to switch theselected sub power storage device can be performed appropriately andsmoothly.

It should be understood that the embodiment disclosed herein isillustrative and non-restrictive in any respect. The scope of thepresent invention is defined by the terms of the claims, rather than thedescription above, and is intended to include any modifications withinthe scope and meaning equivalent to the terms of the claims.

The invention claimed is:
 1. A power supply system for an electricallypowered vehicle equipped with a motor generating power to drive thevehicle, comprising: a main power storage device; an electric powerfeeding line supplying electric power to an inverter that controls anddrives said motor; a first voltage converter provided between saidelectric power feeding line and said main power storage device forconverting voltage therebetween bidirectionally; a plurality of subpower storage devices provided in parallel to each other; a secondvoltage converter provided between said plurality of sub power storagedevices and said electric power feeding line, for converting voltagebetween one of said plurality of sub power storage devices and saidelectric power feeding line bidirectionally; a connection unit providedbetween said plurality of sub power storage devices and said secondvoltage converter, for selectively connecting a sub power storage deviceselected from said plurality of sub power storage devices to said secondvoltage converter; and a switching control device controlling selectiveconnection between said plurality of sub power storage devices and saidsecond voltage converter, said switching control device including aswitching determination unit determining whether or not said selectedsub power storage device should be switched based on states of charge ofsaid plurality of sub power storage devices, a step-up-voltageinstruction unit instructing said first voltage converter to provide avoltage on said electric power feeding line to be a first voltage higherthan a voltage output from said main power storage device and a voltageoutput from a sub power storage device to be connected to said secondvoltage converter after switching, when said switching determinationunit determines that it is necessary to switch said selected sub powerstorage device, a first electric power limiter unit decreasing values ofupper limits on electric power input/output to/from said selected subpower storage device gradually to zero after said voltage on saidelectric power feeding line has reached said first voltage, a connectionswitching control unit switching connection between said plurality ofsub power storage devices and said second voltage converter, when saidfirst electric power limiter unit sets said values of said upper limitson electric power input/output to zero, and a second electric powerlimiter unit increasing said values of said upper limits on electricpower input/output gradually to values corresponding to a state ofcharge of the sub power storage device newly connected to said secondvoltage converter after said connection switching control unit switchesconnection between said plurality of sub power storage devices and saidsecond voltage converter.
 2. The power supply system for an electricallypowered vehicle according to claim 1, wherein said step-up-voltageinstruction unit continues to instruct said first voltage converter toprovide said voltage on said electric power feeding line to be saidfirst voltage until a process for increasing said values of said upperlimits on electric power input/output by said second electric powerlimiter unit ends.
 3. The power supply system for an electricallypowered vehicle according to claim 1, wherein said first voltagecorresponds to a value of an upper limit on said voltage on saidelectric power feeding line controlled by said first voltage converter.4. The power supply system for an electrically powered vehicle accordingto claim 1, wherein said switching control device further includes athird electric power limiter unit temporarily relaxing charging anddischarging limits for said main power storage device in a period fromwhen said first electric power limiter unit starts decreasing saidvalues of said upper limits on electric power input/output to when saidconnection unit completes switching of connection between said pluralityof sub power storage devices and said second voltage converter.
 5. Thepower supply system for an electrically powered vehicle according toclaim 1, wherein said electrically powered vehicle further includes aninternal combustion engine configured to be capable of outputting powerto drive the vehicle independently of said motor, and a travelingcontrol unit starting said internal combustion engine when totalrequired power for said electrically powered vehicle is greater than asum of a value of an upper limit on electric power output from said mainpower storage device and the value of the upper limit on electric poweroutput from said selected sub power storage device.
 6. A method forcontrolling a power supply system for an electrically powered vehicleequipped with a motor generating power to drive the vehicle, said powersupply system including a main power storage device, an electric powerfeeding line supplying electric power to an inverter that controls anddrives said motor, a first voltage converter provided between saidelectric power feeding line and said main power storage device forconverting voltage there between bidirectionally, a plurality of subpower storage devices provided in parallel to each other, a secondvoltage converter provided between said plurality of sub power storagedevices and said electric power feeding line, for converting voltagebetween one of said plurality of sub power storage devices and saidelectric power feeding line bidirectionally, a connection unit providedbetween said plurality of sub power storage devices and said secondvoltage converter, for selectively connecting a sub power storage deviceselected from said plurality of sub power storage devices to said secondvoltage converter, and a switching control device controlling selectiveconnection between said plurality of sub power storage devices and saidsecond voltage converter, said method for controlling comprising thesteps of: determining, with the switching control device, whether or notsaid selected sub power storage device should be switched based onstates of charge of said plurality of sub power storage devices;instructing, with the switching control device, said first voltageconverter to provide a voltage on said electric power feeding line to bea first voltage higher than a voltage output from said main powerstorage device and a voltage output from a sub power storage device tobe connected to said second voltage converter after switching when saidstep of determining determines that it is necessary to switch saidselected sub power storage device; decreasing, with the switchingcontrol device, values of upper limits on electric power input/outputto/from said selected sub power storage device gradually to zero aftersaid voltage on said electric power feeding line has reached said firstvoltage; switching, with the switching control device, connectionbetween said plurality of sub power storage devices and said secondvoltage converter when said step of decreasing sets said values of saidupper limits on electric power input/output to zero; and increasing,with the switching control device, said values of said upper limits onelectric power input/output gradually to values corresponding to a stateof charge of the sub power storage device newly connected to said secondvoltage converter after said step of switching switches connectionbetween said plurality of sub power storage devices and said secondvoltage converter.
 7. The method for controlling a power supply systemfor an electrically powered vehicle according to claim 6, furthercomprising a step of continuing to instruct said first voltage converterto provide said voltage on said electric power feeding line to be saidfirst voltage until a process for increasing said values of said upperlimits on electric power input/output in said step of increasing ends.8. The method for controlling a power supply system for an electricallypowered vehicle according to claim 6, wherein said first voltagecorresponds to a value of an upper limit on said voltage on saidelectric power feeding line controlled by said first voltage converter.9. The method for controlling a power supply system for an electricallypowered vehicle according to claim 6, further comprising a step oftemporarily relaxing charging and discharging limits for said main powerstorage device in a period from when said step of decreasing startsdecreasing said values of said upper limits on electric powerinput/output to when said connection unit completes switching ofconnection between said plurality of sub power storage devices and saidsecond voltage converter.
 10. The method for controlling a power supplysystem for an electrically powered vehicle according to claim 6, whereinsaid electrically powered vehicle is further equipped with an internalcombustion engine configured to be capable of outputting power to drivethe vehicle independently of said motor, and said internal combustionengine is started when a total required power for said electricallypowered vehicle is greater than a sum of a value of an upper limit onelectric power output from said main power storage device and the valueof the upper limit on electric power output from said selected sub powerstorage device.